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Linking
15-213/18-213/15-513: Introduction to Computer Systems 14th Lecture, July 1, 2020
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition 1

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Today
 Linking
▪ Motivation
▪ What it does
▪ How it works
▪ Dynamic linking
 Case study: Library interpositioning
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Example C Program
int sum(int *a, int n);
int array[2] = {1, 2};
int main(int argc, char** argv) {
int val = sum(array, 2);
return val; }
main.c
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition 3
int sum(int *a, int n)
{
int i, s = 0;
for (i = 0; i < n; i++) { s += a[i]; } return s; } sum.c Carnegie Mellon Linking  Programs are translated and linked using a compiler driver: ▪ linux> gcc -Og -o prog main.c sum.c
▪ linux> ./prog
main.c
sum.c
Source files
Translators (cpp, cc1, as)
Translators (cpp, cc1, as)
main.o
Linker (ld)
prog
sum.o
Separately compiled relocatable object files
Fully linked executable object file (contains code and data for all functions defined in main.c and sum.c)
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Why Linkers?
 Reason 1: Modularity
▪ Program can be written as a collection of smaller source files, rather than one monolithic mass.
▪ Can build libraries of common functions (more on this later) ▪ e.g., Math library, standard C library
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Why Linkers? (cont)
 Reason 2: Efficiency
▪ Time: Separate compilation
▪ Change one source file, compile, and then relink. ▪ No need to recompile other source files.
▪ Can compile multiple files concurrently.
▪ Space: Libraries
▪ Common functions can be aggregated into a single file… ▪ Option 1: Static Linking
– Executable files and running memory images contain only the library code they actually use
▪ Option 2: Dynamic linking
– Executable files contain no library code
– During execution, single copy of library code can be shared across all executing processes
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What Do Linkers Do?  Step 1: Symbol resolution
▪ Programs define and reference symbols (global variables and functions):
▪ void swap() {…} ▪ swap();
▪ int *xp = &x;
/* define symbol swap */
/* reference symbol swap */
/* define symbol xp, reference x */
▪ Symbol definitions are stored in object file (by assembler) in symbol table. ▪ Symbol table is an array of entries
▪ Each entry includes name, size, and location of symbol.
▪ During symbol resolution step, the linker associates each symbol reference with exactly one symbol definition.
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Symbols in Example C Program
Definitions
int sum(int *a, int n);
int array[2] = {1, 2};
int main(int argc, char** argv) {
int val = sum(array, 2);
return val; }
main.c
Reference
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition 8
int sum(int *a, int n)
{
int i, s = 0;
for (i = 0; i < n; i++) { s += a[i]; } return s; } sum.c Carnegie Mellon What Do Linkers Do? (cont’d)  Step 2: Relocation ▪ Merges separate code and data sections into single sections ▪ Relocates symbols from their relative locations in the .o files to their final absolute memory locations in the executable. ▪ Updates all references to these symbols to reflect their new positions. Let’s look at these two steps in more detail.... Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition 9 Carnegie Mellon Three Kinds of Object Files (Modules)  Relocatable object file (.o file) ▪ Contains code and data in a form that can be combined with other relocatable object files to form executable object file. ▪ Each .o file is produced from exactly one source (.c) file  Executable object file (a.out file) ▪ Contains code and data in a form that can be copied directly into memory and then executed.  Shared object file (.so file) ▪ Special type of relocatable object file that can be loaded into memory and linked dynamically, at either load time or run-time. ▪ Called Dynamic Link Libraries (DLLs) by Windows Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition 10 Carnegie Mellon Executable and Linkable Format (ELF)  Standard binary format for object files  One unified format for ▪ Relocatable object files (.o), ▪ Executable object files (a.out) ▪ Shared object files (.so)  Generic name: ELF binaries Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition 11 Carnegie Mellon ELF Object File Format  Elfheader ▪ Word size, byte ordering, file type (.o, exec, .so), machine type, etc.  Segmentheadertable ▪ Page size, virtual address memory segments (sections), segment sizes.  .textsection ▪ Code  .rodata section ▪ Read only data: jump tables, string constants, ...  .datasection ▪ Initialized global variables  .bsssection ▪ Uninitialized global variables ▪ “Block Started by Symbol” ▪ “Better Save Space” 0 ELF header Segment header table (required for executables) .text section .rodata section .data section .bss section .symtab section .rel.txt section .rel.data section .debug section Section header table ▪ Has section header but occupies no space Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition 12 Carnegie Mellon ELF Object File Format (cont.)  .symtabsection ▪ Symbol table ▪ Procedure and static variable names ▪ Section names and locations  .rel.textsection ▪ Relocation info for .text section ▪ Addresses of instructions that will need to be modified in the executable ▪ Instructions for modifying  .rel.datasection ▪ Relocation info for .data section ▪ Addresses of pointer data that will need to be modified in the merged executable  .debugsection ▪ Info for symbolic debugging (gcc -g)  Section header table ▪ Offsets and sizes of each section 0 ELF header Segment header table (required for executables) .text section .rodata section .data section .bss section .symtab section .rel.txt section .rel.data section .debug section Section header table Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition 13 Carnegie Mellon Linker Symbols  Global symbols ▪ Symbols defined by module m that can be referenced by other modules. ▪ e.g., non-static C functions and non-static global variables.  External symbols ▪ Global symbols that are referenced by module m but defined by some other module.  Local symbols ▪ Symbols that are defined and referenced exclusively by module m. ▪ e.g, C functions and global variables defined with the static attribute. ▪ Local linker symbols are not local program variables Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition 14 Carnegie Mellon Step 1: Symbol Resolution ...that’s defined here Referencing a global... int sum(int *a, int n); int array[2] = {1, 2}; int main(int argc,char **argv) { int val = sum(array, 2); return val; } main.c int sum(int *a, int n) { } sum.c int i, s = 0; for (i = 0; i < n; i++) { s += a[i]; } return s; Defining a global Referencing a global... ...that’s defined here Linker knows nothing of i or s Linker knows nothing of val Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition 15 Carnegie Mellon Symbol Identification Which of the following names will be in the symbol table of symbols.o? symbols.c: Names: • incr • foo • a • argc • argv • b • main • printf • "O%thde\rns?" Can find this with readelf: linux> readelf –s symbols.o
int incr = 1;
static int foo(int a) {
int b = a + incr;
return b; }
int main(int argc, char* argv[]) {
printf(“%d\n”, foo(5));
return 0; }
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Local Symbols
 Local non-static C variables vs. local static C variables ▪ Local non-static C variables: stored on the stack
▪ Local static C variables: stored in either .bss or .data
static int x = 15;
int f() {
static int x = 17; return x++;
}
int g() {
static int x = 19;
return x += 14;
}
int h() {
return x += 27;
}
Compiler allocates space in .data for each definition of x
Creates local symbols in the symbol table with unique names, e.g., x, x.1721 and x.1724.
static-local.c
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How Linker Resolves Duplicate Symbol Definitions
 Program symbols are either strong or weak ▪ Strong: procedures and initialized globals
▪ Weak: uninitialized globals
▪ Or ones declared with specifier extern
strong weak
p1.c
p2.c
int foo=5;
p1() { }
int foo;
p2() { }
strong
strong
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Linker’s Symbol Rules
 Rule 1: Multiple strong symbols are not allowed ▪ Each item can be defined only once
▪ Otherwise: Linker error
 Rule 2: Given a strong symbol and multiple weak symbols, choose the strong symbol
▪ References to the weak symbol resolve to the strong symbol
 Rule 3: If there are multiple weak symbols, pick an arbitrary one
▪ Can override this with gcc –fno-common
 Puzzles on the next slide
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Linker Puzzles
p1() {}
Link time error: two strong symbols (p1)
int x; p1() {}
int x; p1() {}
int x; p2() {}
References to x will refer to the same uninitialized int. Is this what you really want?
int x;
int y;
p1() {}
double x;
p2() {}
Writes to x in p2 might overwrite y! Evil!
int x=7; int y=5; p1() {}
double x; p2() {}
Writes to x in p2 might overwrite y! Nasty!
References to x will refer to the same initialized variable.
Important: Linker does not do type checking.
int x=7;
p1() {}
int x; p2() {}
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Type Mismatch Example
long int x; /* Weak symbol */
int main(int argc,
char *argv[]) {
printf(“%ld\n”, x);
return 0; }
mismatch-main.c
/* Global strong symbol */
/* Global strong symbol */
double x = 3.14;
double x = 3.14;
mismatch-variable.c
 Compiles without any errors or warnings  What gets printed?
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Global Variables  Avoid if you can
 Otherwise
▪ Use static if you can
▪ Initialize if you define a global variable
▪ Use extern if you reference an external global variable
▪ Treated as weak symbol
▪ But also causes linker error if not defined in some file
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Use of extern in .h Files (#1)
c1.c
#include “global.h”
int f() {
return g+1;
}
c2.c
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition 23
global.h
extern int g;
int f();
#include
#include “global.h”
int g = 0;
int main(int argc, char argv[]) { int t = f();
printf(“Calling f yields %d\n”, t); return 0;
}

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Linking Example
int sum(int *a, int n); int array[2] = {1, 2};
int main(int argc,char **argv)
{
int val = sum(array, 2); return val;
}
main.c
int sum(int *a, int n)
{
}
sum.c
int i, s = 0;
for (i = 0; i < n; i++) { s += a[i]; } return s; Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition 25 Carnegie Mellon Step 2: Relocation Relocatable Object Files Executable Object File System code System data .text .data 0 Headers System code main() sum() More system code System data int array[2]={1,2} .symtab .debug main.o .text main() int array[2]={1,2} sum.o sum() .text .data .text .data Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition 26 Carnegie Mellon Relocation Entries int array[2] = {1, 2}; int main(int argc, char** argv) { int val = sum(array, 2); return val; } main.c 0000000000000000

:
0: 48 83 ec 08
sub $0x8,%rsp
mov $0x2,%esi
mov $0x0,%edi # %edi = &array
a: R_X86_64_32 array # Relocation entry
callq 13 # sum()
f: R_X86_64_PC32 sum-0x4 # Relocation entry
4: be 9: bf
e: e8
13: 48 17: c3
02 00 00 00 00 00 00 00
00 00 00 00
83 c4 08
add $0x8,%rsp retq
main.o
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Source: objdump –r –d main.o 27

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Relocated .text section
callq instruction uses PC-relative addressing for sum():
0x4004e8 = 0x4004e3 + 0x5
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition Source: objdump -d prog 28
00000000004004d0

: 4004d0: 48 83 ec 08 4004d4: be 02 00 00 00 4004d9: bf 18 10 60 00
sub $0x8,%rsp
mov $0x2,%esi
mov $0x601018,%edi # %edi = &array
4004de: e8 05 00 00 4004e3: 48 83 c4 08 4004e7: c3
00000000004004e8 :
4004e8: b8 00 4004ed: ba 00 4004f2: eb 09 4004f4: 48 63 4004f7: 03 04 4004fa: 83 c2 01 4004fd: 39 f2 4004ff: 7c f3 400501: f3 c3
00
00 00
callq 4004e8
add $0x8,%rsp
retq
# sum()
00 00 00 00
ca 8f
mov $0x0,%eax
mov $0x0,%edx
jmp 4004fd movslq %edx,%rcx
add (%rdi,%rcx,4),%eax add $0x1,%edx
cmp %esi,%edx
jl 4004f4 repz retq

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Loading Executable Object Files
Memory invisible to user code
%rsp
(stack pointer)
Kernel virtual memory
User stack (created at runtime)
Memory-mapped region for shared libraries
Run-time heap (created by malloc)
Read/write data segment (.data, .bss)
Read-only code segment (.init, .text, .rodata)
Unused
Executable Object File
0
ELF header
Program header table (required for executables)
.init section
.text section
.rodata section
.data section
.bss section
.symtab
.debug
.line
.strtab
Section header table (required for relocatables)
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition 0
29
0x400000
brk
Loaded from
the executable file

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Quiz Time!
Check out:
https://canvas.cmu.edu/courses/16836
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Libraries: Packaging a Set of Functions
 How to package functions commonly used by programmers?
▪ Math, I/O, memory management, string manipulation, etc.
 Awkward, given the linker framework so far: ▪ Option 1: Put all functions into a single source file
▪ Programmers link big object file into their programs
▪ Space and time inefficient
▪ Option 2: Put each function in a separate source file
▪ Programmers explicitly link appropriate binaries into their programs
▪ More efficient, but burdensome on the programmer
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Old-Fashioned Solution: Static Libraries  Static libraries (.a archive files)
▪ Concatenate related relocatable object files into a single file with an index (called an archive).
▪ Enhance linker so that it tries to resolve unresolved external references by looking for the symbols in one or more archives.
▪ If an archive member file resolves reference, link it into the executable.
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Creating Static Libraries
atoi.c printf.c
Translator Translator atoi.o printf.o
Archiver (ar)
libc.a

random.c
Translator
random.o
unix> ar rs libc.a \
atoi.o printf.o … random.o
C standard library
 Archiverallowsincrementalupdates
 Recompile function that changes and replace .o file in archive.
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Commonly Used Libraries
libc.a (the C standard library)
▪ 4.6 MB archive of 1496 object files.
▪ I/O, memory allocation, signal handling, string handling, data and time, random numbers, integer math
libm.a (the C math library)
▪ 2 MB archive of 444 object files.
▪ floating point math (sin, cos, tan, log, exp, sqrt, …)
% ar –t /usr/lib/libc.a | sort

fork.o

fprintf.o fpu_control.o fputc.o freopen.o fscanf.o fseek.o fstab.o

Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition 34
% ar –t /usr/lib/libm.a | sort

e_acos.o
e_acosf.o
e_acosh.o e_acoshf.o e_acoshl.o e_acosl.o e_asin.o e_asinf.o e_asinl.o …

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Linking with Static Libraries
#include
#include “vector.h”
int x[2] = {1, 2};
int y[2] = {3, 4};
int z[2];
int main(int argc, char**
argv)
{
addvec(x, y, z, 2);
printf(“z = [%d %d]\n”,
z[0], z[1]);
return 0; }
main2.c
}
z[i] = x[i] + y[i];
addvec.c
void multvec(int *x, int *y, int *z, int n)
{
int i;
for (i = 0; i < n; i++) z[i] = x[i] * y[i]; } multvec.c Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition 35 libvector.a void addvec(int *x, int *y, int *z, int n) { int i; for (i = 0; i < n; i++) Carnegie Mellon Linking with Static Libraries addvec.o multvec.o main2.c vector.h Archiver (ar) Translators (cpp, cc1, as) Relocatable main2.o object files libvector.a libc.a addvec.o printf.o and any other Static libraries Linker (ld) prog2c modules called by printf.o unix> gcc –static –o prog2c \
main2.o -L. -lvector
Fully linked executable object file (861,232 bytes)
“c” for “compile-time”
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Using Static Libraries
 Linker’s algorithm for resolving external references:
▪ Scan .o files and .a files in the command line order.
▪ During the scan, keep a list of the current unresolved references.
▪ As each new .o or .a file, obj, is encountered, try to resolve each unresolved reference in the list against the symbols defined in obj.
▪ If any entries in the unresolved list at end of scan, then error.
 Problem:
▪ Command line order matters!
▪ Moral: put libraries at the end of the command line.
unix> gcc -static -o prog2c -L. -lvector main2.o main2.o: In function `main’:
main2.c:(.text+0x19): undefined reference to `addvec’ collect2: error: ld returned 1 exit status
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Modern Solution: Shared Libraries
 Static libraries have the following disadvantages:
▪ Duplication in the stored executables (every function needs libc)
▪ Duplication in the running executables
▪ Minor bug fixes of system libraries require each application to explicitly
relink
▪ Rebuild everything with glibc?
▪ https://security.googleblog.com/2016/02/cve-2015-7547-glibc- getaddrinfo-stack.html
 Modern solution: shared libraries
▪ Object files that contain code and data that are loaded and linked into
an application dynamically, at either load-time or run-time
▪ Also called: dynamic link libraries, DLLs, .so files
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Shared Libraries (cont.)
 Dynamic linking can occur when executable is first loaded and run (load-time linking)
▪ Common case for Linux, handled automatically by the dynamic linker (ld-linux.so)
▪ Standard C library (libc.so) usually dynamically linked
 Dynamic linking can also occur after program has begun (run-time linking)
▪ In Linux, this is done by calls to the dlopen() interface ▪ Distributing software
▪ High-performance web servers
▪ Runtime library interpositioning
 Shared library routines can be shared by multiple processes ▪ More on this when we learn about virtual memory
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What dynamic libraries are required?  .interpsection
▪ Specifies the dynamic linker to use (i.e., ld-linux.so)  .dynamicsection
▪ Specifies the names, etc of the dynamic libraries to use
▪ Follow an example of prog
(NEEDED) Shared library: [libm.so.6]
 Where are the libraries found? ▪ Use “ldd” to find out:
unix> ldd prog
linux-vdso.so.1 => (0x00007ffcf2998000)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f99ad927000) /lib64/ld-linux-x86-64.so.2 (0x00007f99adcef000)
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Dynamic Library Example
addvec.c multvec.c
unix> gcc –Og –c addvec.c multvec.c -fpic
Translator Translator addvec.o multvec.o
Loader (ld)
libvector.so
unix> gcc -shared -o libvector.so \
addvec.o multvec.o
Dynamic vector library
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Dynamic Linking at Load-time
main2.c vector.h
unix> gcc -shared -o libvector.so \
addvec.c multvec.c -fpic
libc.so
libvector.so
Relocation and symbol table info
unix> gcc –o prog2l \
main2.o ./libvector.so
libc.so
libvector.so
Code and data
Relocatable object file
Partially linked executable object file (8488 bytes)
main2.o
Linker (ld) prog2l
Fully linked
Dynamic linker (ld-linux.so)
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition 42
executable
in memory
Translators (cpp, cc1, as)
Loader (execve)

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Dynamic Linking at Run-time
#include #include #include int x[2] = {1, 2};
int y[2] = {3, 4};
int z[2];
int main(int argc, char** argv)
{
void *handle;
void (*addvec)(int *, int *, int *, int); char *error;
/* Dynamically load the shared library that contains addvec() */
handle = dlopen(“./libvector.so”, RTLD_LAZY); if (!handle) {
fprintf(stderr, “%s\n”, dlerror());
exit(1); }
. . .
dll.c
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Dynamic Linking at Run-time (cont’d)

/* Get a pointer to the addvec() function we just loaded */
addvec = dlsym(handle, “addvec”);
if ((error = dlerror()) != NULL) {
fprintf(stderr, “%s\n”, error);
exit(1); }
/* Now we can call addvec() just like any other function */
addvec(x, y, z, 2);
printf(“z = [%d %d]\n”, z[0], z[1]);
/* Unload the shared library */
if (dlclose(handle) < 0) { fprintf(stderr, "%s\n", dlerror()); exit(1); } return 0; } dll.c Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition 44 Carnegie Mellon Dynamic Linking at Run-time dll.c vector.h unix> gcc -shared -o libvector.so \
addvec.c multvec.c -fpic
Translators (cpp, cc1, as)
prog2r
dll.o -ldl
libc.so
Code and data
libc.so
libvector.so
Relocatable object file
dll.o
Relocation and symbol table info
Linker (ld)
unix> gcc -rdynamic –o prog2r \
Partially linked executable object file (8784 bytes)
Fully linked executable in memory
Loader (execve)
Dynamic linker (ld-linux.so)
Call to dynamic linker via dlopen
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Linking Summary
 Linking is a technique that allows programs to be constructed from multiple object files
 Linking can happen at different times in a program’s lifetime:
▪ Compile time (when a program is compiled)
▪ Load time (when a program is loaded into memory) ▪ Run time (while a program is executing)
 Understanding linking can help you avoid nasty errors and make you a better programmer
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Today
 Linking
 Case study: Library interpositioning
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Case Study: Library Interpositioning  Documented in Section 7.13 of book
 Library interpositioning: powerful linking technique that allows programmers to intercept calls to arbitrary functions
 Interpositioning can occur at:
▪ Compile time: When the source code is compiled
▪ Link time: When the relocatable object files are statically linked to form an executable object file
▪ Load/run time: When an executable object file is loaded into memory, dynamically linked, and then executed.
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Some Interpositioning Applications
 Security
▪ Confinement (sandboxing)
▪ Behind the scenes encryption
 Debugging
▪ In 2014, two Facebook engineers debugged a treacherous 1-year
old bug in their iPhone app using interpositioning
▪ Code in the SPDY networking stack was writing to the wrong location
▪ Solved by intercepting calls to Posix write functions (write, writev, pwrite)
Source: Facebook engineering blog post at:
https://code.facebook.com/posts/313033472212144/debugging-file-corruption-on-ios/
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Some Interpositioning Applications
 Monitoring and Profiling
▪ Count number of calls to functions
▪ Characterize call sites and arguments to functions ▪ Malloc tracing
▪ Detecting memory leaks
▪ Generating address traces
 Error Checking
▪ C Programming Lab used customized versions of malloc/free to do
careful error checking
▪ Other labs (malloc, shell, proxy) also use interpositioning to enhance checking capabilities
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Example program
#include
#include
#include
int main(int argc,
char *argv[])
{
int i;
for (i = 1; i < argc; i++) { void *p = malloc(atoi(argv[i])); free(p); } return(0); } int.c Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition 51  Goal: trace the addresses and sizes of the allocated and freed blocks, without breaking the program, and without modifying the source code. Three solutions: interpose on the library malloc and free functions at compile time, link time, and load/run time.  Carnegie Mellon Compile-time Interpositioning #ifdef COMPILETIME #include #include
/* malloc wrapper function */
void *mymalloc(size_t size)
{
void *ptr = malloc(size); printf(“malloc(%d)=%p\n”, (int)size, ptr); return ptr;
}
/* free wrapper function */
void myfree(void *ptr)
{
free(ptr);
printf(“free(%p)\n”, ptr);
}
#endif
mymalloc.c
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition 52

Carnegie Mellon
Compile-time Interpositioning
#define malloc(size) mymalloc(size) #define free(ptr) myfree(ptr)
void *mymalloc(size_t size);
void myfree(void *ptr);
malloc.h
linux> make intc
gcc -Wall -DCOMPILETIME -c mymalloc.c gcc -Wall -I. -o intc int.c mymalloc.o linux> make runc
./intc 10 100 1000 malloc(10)=0x1ba7010
free(0x1ba7010) malloc(100)=0x1ba7030 free(0x1ba7030) malloc(1000)=0x1ba70a0 free(0x1ba70a0) linux>
Search for leads to /usr/include/malloc.h
Search for leads to
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition 53

Carnegie Mellon
Link-time Interpositioning
#ifdef LINKTIME
#include
void *__real_malloc(size_t size); void __real_free(void *ptr);
/* malloc wrapper function */
void *__wrap_malloc(size_t size)
{
void *ptr = __real_malloc(size); /* Call libc malloc */ printf(“malloc(%d) = %p\n”, (int)size, ptr);
return ptr;
}
/* free wrapper function */
void __wrap_free(void *ptr)
{
__real_free(ptr); /* Call libc free */
printf(“free(%p)\n”, ptr);
}
#endif
mymalloc.c
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
54

Carnegie Mellon
Link-time Interpositioning
 The “-Wl” flag passes argument to linker, replacing each comma with a space.
 The “–wrap,malloc ” arg instructs linker to resolve references in a special way:
▪ Refs to malloc should be resolved as __wrap_malloc
▪ Refs to __real_malloc should be resolved as malloc
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition 55
linux> make intl
gcc -Wall -DLINKTIME -c mymalloc.c
gcc -Wall -c int.c
gcc -Wall -Wl,–wrap,malloc -Wl,–wrap,free -o intl \
int.o mymalloc.o
linux> make runl
./intl 10 100 1000 malloc(10) = 0x91a010 free(0x91a010)
. ..
Search for leads to /usr/include/malloc.h

#ifdef RUNTIME #define _GNU_SOURCE #include #include #include /* malloc wrapper function */
void *malloc(size_t size)
{
}
mymalloc.c
Observe that DON’T have
#include
void *(*mallocp)(size_t size);
char *error;
mallocp = dlsym(RTLD_NEXT, “malloc”); /* Get addr of libc malloc */ if ((error = dlerror()) != NULL) {
fputs(error, stderr);
exit(1); }
char *ptr = mallocp(size); /* Call libc malloc */ printf(“malloc(%d) = %p\n”, (int)size, ptr); return ptr;
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition 56
Load/Run-time
Interpositioning
Carnegie Mellon

Carnegie Mellon
Load/Run-time Interpositioning
/* free wrapper function */
void free(void *ptr) {
void (*freep)(void *) = NULL;
char *error;
if (!ptr)
return;
freep = dlsym(RTLD_NEXT, “free”); /* Get address of libc free */ if ((error = dlerror()) != NULL) {
fputs(error, stderr);
exit(1); }
freep(ptr); /* Call libc free */
printf(“free(%p)\n”, ptr);
}
#endif
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
57
mymalloc.c

Carnegie Mellon
Load/Run-time Interpositioning
 The LD_PRELOAD environment variable tells the dynamic linker to resolve unresolved refs (e.g., to malloc)by looking in mymalloc.so first.
 Type into (some) shells as:
env LD_PRELOAD=./mymalloc.so ./intr 10 100 1000)
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition 58
linux> make intr
gcc -Wall -DRUNTIME -shared -fpic -o mymalloc.so mymalloc.c -ldl gcc -Wall -o intr int.c
linux> make runr
(LD_PRELOAD=”./mymalloc.so” ./intr 10 100 1000)
malloc(10) = 0x91a010 free(0x91a010)
. ..
linux>
Search for leads to /usr/include/malloc.h

Carnegie Mellon
Interpositioning Recap  Compile Time
▪ Apparent calls to malloc/free get macro-expanded into calls to mymalloc/myfree
▪ Simple approach. Must have access to source & recompile
 Link Time
▪ Use linker trick to have special name resolutions
▪ malloc →__wrap_malloc ▪ __real_malloc→malloc
 Load/Run Time
▪ Implement custom version of malloc/free that use dynamic
linking to load library malloc/free under different names
▪ Can use with ANY dynamically linked binary
env LD_PRELOAD=./mymalloc.so gcc –c int.c)
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition 59

Carnegie Mellon
Linking Recap
 Usually: Just happens, no big deal
 Sometimes: Strange errors
▪ Bad symbol resolution
▪ Ordering dependence of linked .o, .a, and .so files
 For power users:
▪ Interpositioning to trace programs with & without source
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition 60